It is known to provide a milling installation which employs a through-flow (continuous, open-circuit) mill, generally a tubular ball mill, having an inlet end for the material to be ground or comminuted and discharge end for the finished product, the interior of the mill being provided with balls or the like which, by abrasion, impact, compression and a combinations thereof, upon rotation of the tubular housing of the mill, cause size reduction of the material traversing the mill.
Reference will be made hereinafter to a circulating (continuous, closed-circuit) mill system and to a through-flow (single pass) milling system and it is of interest to review the distinctions between them.
In a single-pass continuous (open-circuit) milling system, the raw material is introduced at the inlet end to the mill and is maintained therein for a period (residence time) sufficient to give the final product fineness which is desired, the product being discharged at the outlet end.
In a circulating milling system, the product is a mixture of the desired fines and a recycled coarse component which must be remilled. The product from the discharge side of the mill is conveyed, e,g. by a trough, elevator, bucket conveyor or the like, to the classifier which separates the output mixture into the product fines and the coarse component. From the classifier (sifter) the coarse component is recycled to the inlet of the mill where it is combined with the raw material and reintroduced with the latter into the mill.
While we have referred to a tube-type ball mill above, it should be noted that the invention and the principles described are applicable to all types of mills in which the output rate is a function of the residence time of material in the mill. Such mills include oscillating, tumbling or vibratory mills and like systems.
In single-pass mills and circulating mills of the type described, significant fluctuations in the characteristics of the raw material, with constant milling conditions, result in a change in the fineness of the milled output or product.
If a constant product fineness is desired, in spite of these fluctuations, one modifies the rate at which the raw material or fresh material is introduced into the mill and, in the case of a circulatory milling installation, the classifier setting, i.e. the ratio between fine stream and the coarse stream.
For automatic control various systems have been proposed without being fully satisfactory.
A conventional control system measures the quantity ahead of and behind the sifter or classifier or determines the fineness of these streams and controls, upon a deviation one of these measured values from a set point value, the feed to the mill and partially the fineness setting if the classifier or sifter (see German patent document--Open Application--Offenlegungsschrift Nos. 1,507,483 and 1,507,490).
Other control systems of the prior art utilize the rate of operation of the bucket conveyor or elevator or the drive motor of the mill (see German patent document--Pat. No. 284,154 and German patent document--Open Application--Offenlegungsschrift--No. 1,763,432) to control the feed of material to the mill.
These prior-art control techniques have two distinct disadvantages:
A. The measurement and control parameters have different time functions. The response time is relatively great and is of the order of the residence time of the material in the mill.
B. The flow characteristics of the comminuted material amplifies the fluctuation of the fineness of the milled product with variations in the milling characteristics of the material. As the milled product becomes coarser, for example, because the raw material is less millable, the flowability of the product improves. With a constant discharge geometry of the mill, the increasing flowability results in an increased product discharge and a reduced residence time of the milled material in the mill. This, in turn, reduces the fineness of the product.
The two effects act counter to one another and fluctuations in the fineness are noticeable in all systems in which these effects are pronounced.
If efforts are made to obtain a constant fineness of the product stream, the classifier must be adjusted so as to reduce the introduction of the raw material to a more significant degree than would be ordinarily deemed necessary as a result of changes in the grindability (e.g. A.S.T. or D-408).
Mention can be made of still another control technique in which, upon an increase in the coarseness of the discharged product, a portion of this product, to increase the fineness, is returned to the mill inlet (see German patent document--printed application--Auslegeschrift No.--1,913,440). This system, however, also has the disadvantage that the response time between the detection of a deviation of the fineness from a set point value, through the adjustment of the recycler, to the correction of the fineness is relatively long and unsatisfactory.